Attaching metallic lithium to the negative electrode of a nonaqueous secondary battery has been proposed to obtain improved overdischarge characteristics. For example, JP-A-5-144472, JP-A-5-144473, and JP-A-7-94211 disclose a nonaqueous secondary battery consisting of a spiral wound roll of a positive plate of a lithium-containing complex oxide of a transition metal and a negative plate of a carbon material separated by a separator, in which the negative electrode has a metallic lithium foil adhered to its outermost or peripheral part on the side not facing the positive plate.
The disclosed negative electrode has the active material exposed on its outermost surface that is in contact with a nonaqueous electrolyte. Therefore, the active material particles are apt to fall off through repetition of expansion and contraction accompanying intercalation and deintercalation of lithium ions. As a result, a battery using the negative electrode tends to have a reduced cycle life. Moreover, since metallic lithium is also exposed on the outermost surface of the negative electrode, there is the danger that lithium grows dendritically, and the formed dendrites ultimately fall off the negative electrode or penetrate the separator and then contact with the positive electrode, which causes an internal shortage or ignition.
A trace amount of water can often enter a nonaqueous secondary battery during the production processes. In a nonaqueous secondary battery, water reacts with the nonaqueous electrolyte to decompose it. It has hence been suggested to reduce the water content of a nonaqueous secondary battery thereby to improve charge/discharge cycle characteristics (see JP-A-2001-223030). However, a good deal of time and effort would be involved to reduce the water content to a satisfactory level, which is not economically feasible.
Apart from water, a trace amount of oxygen is unavoidably present in the current collector and the active material. Oxygen forms a compound with lithium during a charge or discharge. Formation of the lithium compound results in a reduction in the amount of reversibly available lithium, namely, an increase of irreversible capacity, because an Li—O bond has a relatively high bonding strength.